Spinal fusion is a common surgical procedure used to correct numerous disease states including degenerative disorders, trauma, instability, and deformity. A frequent method of fusion entails the use of bone screws placed through various sections of the vertebral body including the body, pedicle, facets, lamina, lateral masses, and/or transverse processes. These screws are then linked rigidly with a rod, plate or other fixation device to immobilize the vertebral segments.
Due to the variation in a patient's anatomy and differences in screw placement technique, screws are often not perfectly aligned which makes securement of a fixation device more difficult. To solve this, many screws that have a threaded shank portion incorporate an articulating tulip or receiver connected to the proximal end of the shank portion, such as in a polyaxial or multi-axial bone screw. Polyaxial bone screws allow for a variation in the angulation of the tulip/receiver relative to the shank portion in order to allow the tulip/receiver to more closely align for receiving a fixation device such as a fixation rod within the tulip/receiver. Some bone screws allow for the lateral translation of the tulip/receiver relative to its point of fixation. Further alignment may be accomplished by contouring of the fixation device itself to compensate for any remaining misalignment. For example, if a fixation rod is employed, the rod is bent to conform to the patient anatomy and location of the tulip/receiver to securely attach thereto.
A body in three-dimensional space has six degrees of freedom, namely, translation through the perpendicular x, y, and z planes, combined with the rotation through the three perpendicular axes (pitch, yaw, and roll). Typical articulating polyaxial screws allow three dimensional rotations (pitch, yaw, and roll). Some designs also incorporate lateral x-plane translation. Longitudinal translation (y-plane), generally along the cephalad-caudal direction or axis of the fixating rod or plate, is usually accomplished by fixing the tulip/receiver to different positions along the rod or plate.
Anterior/posterior translation (along the z-plane) is typically accomplished by persuading the vertebral body itself, using instruments to raise or lower the vertebral body until the tulip/receiver is properly aligned with the rod or plate. Frequently, however, this anterior/posterior translation may not be desirable as it may produce suboptimal alignment of the vertebral bodies or even cause fractures of the bone or pullout of the shank portion of the screw from the bone due to the stresses placed on it during the persuading process. The other option for adjustment along the z-axis employed is to partially back out the screw, leaving it proud. This, however, reduces the bone-screw interface thereby weakening the overall strength of the construct. Some designs, such as the one illustrated in U.S. Pat. No. 7,588,593, allow for vertical adjustment but require manual assembly of the screw and head construct during surgery. Hence, there is a need for modular bone screw assemblies that can provide variable angle orientation together with z-axis translation which are easy to assemble.